Contact lenses having a toric optical zone (commonly referred to as “toric contact lenses”) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for astigmatism. Since astigmatism that requires vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. A toric surface is formed on either the posterior lens surface (to achieve a “back surface toric lens”) or the anterior lens surface (to form a “front surface toric lens”).
Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have a stabilization structure to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism. For example, a stabilization structure may comprise one or more sections of the lens periphery which are thicker (or thinner) than other sections to provide the stabilization. For example, a ballast is a thick portion that will assume a downward position when a lens is inserted in an eye, and the ballast axis of a lens extends in the up and down direction when a lens assumes its position on the eye. Other stabilization structures are known in the art. Regardless of the structure of the stabilization structure, the axis that extends in the up and down direction when a lens assumes its position on the eye will be referred to herein as the ballast axis.
Toric contact lenses are manufactured with a selected relationship (referred to herein as offset) between the cylindrical axis of the toric optical zone and the orientation of the stabilization structure. This relationship is expressed as a number of degrees that the cylindrical axis is offset from the ballast axis. Accordingly, toric contact lens prescriptions specify offset, with toric lenses generally being offered in 5 or 10-degree increments ranging from 0 degrees to 180 degrees.
In summary, a prescription for a toric contact lens will typically specify spherical correction, cylindrical correction and offset to define the optical correction, as well as an overall lens diameter and base curve to define fitting parameters.
In prescribing a patient with a toric contact lens, practitioners conventionally use sets of diagnostic contact lenses that consist of a selected subset of a series of lenses, to determine which lens in the series will provide appropriate fitting characteristics and optical correction. For example, a series may comprise lenses of a common spherical power but different cylindrical powers.
It is desirable that lenses in such a series (including the lenses of a fitting set) have consistent fitting characteristics over the range of optical corrections of the series. However, a problem with toric contact lenses is that, in order to provide a series of toric lenses over a range of cylindrical corrections, thicknesses of the lenses in the series will typically vary considerably based on cylindrical correction; for example, for a series in which lenses have a common spherical power, lenses having higher cylindrical correction will have greater thicknesses in at least one portion of the lenses. This ultimately results in lenses in the series having different fitting characteristics, thereby affecting, for example on-eye comfort or lens stability at certain cylindrical corrections.
Thus, a lens series which provides consistent fitting characteristics over a range of spherical and cylindrical corrections is highly desirable. The term “set” will be used herein below to refer to a complete series or a subset thereof.